Receiving unit receiving digital television broadcasting

Abstract

In a receiving unit, an AGC circuit controls the gain of a high frequency amplifier based on an output signal of a mixer, that is, an intermediate frequency signal obtained before an unnecessary frequency component is removed by a BPF. Therefore, deterioration in the distortion characteristic of the mixer can be suppressed more effectively than a conventional case where the gain of the high frequency amplifier is controlled based on an intermediate frequency signal that has passed through the BPF and amplified by an intermediate frequency amplifier.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a receiving unit, and in particular, to a receiving unit receiving digital television broadcasting.[0003]2. Description of the Background Art[0004]Conventionally, in a receiving unit receiving analog television broadcasting, a high frequency signal received by an antenna is amplified by a high frequency amplifier, the amplified high frequency signal is converted into an intermediate frequency signal by a mixer, an unnecessary frequency component in the generated intermediate frequency signal is attenuated by a band pass filter (BPF), the intermediate frequency signal that has passed through the BPF is amplified by an intermediate frequency amplifier, a desired frequency component in the amplified intermediate frequency signal is extracted by a surface acoustic wave filter, the extracted intermediate frequency signal is demodulated by a demodulator, and the gain of the high frequency ampl...

AI Technical Summary

Benefits of technology

[0007]Accordingly, a main object of the present invention is to provide a receiving unit capable of fully suppressing deterioration in the distortion characteristic of a mixer.

[0009]Therefore, deterioration in the distortion characteristic of the mixer can be suppressed more effectively than a conventional case where the gain of the high frequency amplifier is controlled based on an output signal of the intermediate frequency amplifier.

[0013]In this instance, when the gain of the high frequency amplifier is controlled based on the intermediate frequency signal obtained before the unnecessary frequency component is attenuated by the first filter, and as a result, the gain of the high frequency amplifier is lowered too much, the gain of the high frequency amplifier can be increased by controlling the gain of the high frequency amplifier based on the intermediate frequency signal amplified by the intermediate frequency amplifier. In contrast, when the gain of the high frequency amplifier is controlled based on the intermediate frequency signal amplified by the intermediate frequency amplifier, and as a result, the distortion characteristic of the mixer is deteriorated, deterioration in the distortion characteristic of the mixer can be suppressed effectively by controlling the gain of the high frequency amplifier based on the intermediate frequency signal obtained before the unnecessary frequency component is attenuated by the first filter.

Problems solved by technology

In a conventional receiving unit, however, a disturbance wave component adjacent to a desired channel is attenuated by a BPF for intermediate frequency, and thus the disturbance wave component is not fully detected at a subsequent stage thereof, resulting in unsatisfactory gain control by an AGC circuit.

Therefore, deterioration in the distortion characteristic of a mixer cannot be fully suppressed, causing a problem that a disturbance wave adjacent to a desired channel is generated adjacent to an intermediate frequency band, and a harmonic generated by the distortion in the mixer interferes with a desired intermediate frequency signal.

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